In the great variety of coating compositions that contain chromic acid or its equivalents, it has been known that it may be desirable to modify or control the pH of the coating composition. For example, in U.S. Pat. No. 3,630,789 a treating solution for metal substrates containing chromic acid is formulated with careful control of the treating solution pH. Such pH should be about 1.8-5 to prevent the reaction of composition ingredients before application, while maintaining a solution bath that will effectively treat base metals.
In the variety of chrome-containing coating compositions, some have been referred to as bonding compositions, as in U.S. Pat. No. 3,382,081. It occasionally has been found desirable in these bonding compositions containing chromic acid along with a reducing agent for the chromium, or in compositions containing chrome in reduced form, to regulate the resulting coating bath pH during the coating operation. Such regulation will typically maintain the bath at a pH of about 3 or above. In U.S. Pat. No. 2,911,332 a bath of chromic acid and reducing agent is shown. When such is used in coating tin-plated steel, it has been taught to be helpful for the bath pH to be in the 3 to 10 range. In U.S. Pat. No. 3,553,034 the chromic acid in the bath is partly reduced to prepare a passivating solution for treating zinc. This solution is taught to be more effective during the treating process if the bath pH is maintained within the range of from 2.8 to 3.8.
One development in chrome-containing compositions includes formulation with pulverulent metal, especially pulverulent zinc. Thus, U.S. Pat. No. 3,671,331 discloses employing finely divided zinc most particularly in bonding coatings, i.e., compositions containing hexavalent chromium providing substance, such as chromic acid, and an agent for reducing the hexavalent chromium provided by the chromic acid. Further, U.S. Pat. No. 3,697,738 discloses the development of a coating composition that may contain, as principal ingredients, pulverulent zinc plus chromic acid.
These ingredients when present at the point of use of the resulting coating composition, may be readily blended together to prepare the composition for such use. However, it is not unusual to prepare the composition for subsequent storage, or for shipment before use. In such case, to prevent deleterious reaction or gelation of composition constituents during storage and/or shipment, it has been necessary to pre-package the coating composition ingredients in separate components. One of these components contains the finely divided metal; an additional component contains typically chromic acid or its equivalent and may further contain reducing agent for the hexavalent chromium. A third package is usually provided that contains one or more agents such as thickening agents, dispersing agents or suspending agents that will provide a final blended coating formulation of augmented stability and uniformity.